/Engine/Source/Runtime/Core/Public/Containers/UnrealString.h
Just like Printf, but appends the formatted text to the existingFStringinstead.
a reference to the modified string, so that it can be chained
template<typename FmtType, typename... Types> [FString](API\Runtime\Core\Containers\FString) & Appendf ( const FmtType & Fmt, Types... Args )